The farthest rings of Uranus offer a stunning spectacle: one shines with icy blue, the other with dusty red. These hues reveal distinctly different compositions: water ice on one side, carbon-loaded dust on the other.
Unlike the famous rings of Saturn, those of Uranus are faint and were discovered by chance in 1977 during a stellar occultation. The two outermost rings, named mu and nu, were spotted between 2003 and 2005 by Mark Showalter's team. Their chromatic duality has kept astronomers busy ever since.
An image of Uranus taken by the NIRCam camera of the James Webb Space Telescope (NASA/ESA/CSA) shows the planet and its rings.
Credit: NASA, ESA, CSA, STScI
New infrared data from the James Webb Telescope have yielded the first complete spectrum of the mu and nu rings. Imke de Pater, from the University of California, Berkeley, led the study. According to her, by analyzing the light reflected by these rings, one can determine the size of the particles and their composition. This information helps understand the formation of the Uranian system. The results confirm the blue and red hues already observed.
The mu ring stands out for its blue color, a sign of a water ice composition. This characteristic recalls Saturn's E ring, fed by geysers from the moon Enceladus. For Uranus, the icy particles come from Mab, a small moon only 7.5 miles (12 kilometers) in diameter, discovered by Mark Showalter in 2003. But why is Mab primarily composed of ice while the other small inner moons are rocky? This question remains unanswered for now.
The nu ring, on the other hand, shows a red hue due to a high proportion of carbonaceous dust. Its spectrum reveals 10 to 15% organic compounds, typical of cold environments in the Solar System. These particles likely come from micrometeorite impacts on still-unknown moons located between known satellites. Imke de Pater wonders about the compositional difference between the parent bodies of the two rings. Future observations should help settle the matter.
Uranus's inner moons, including Mab (top).
Credit: NASA/ESA/CSA/STScI/M. El Moutamid (SWRI)/M. Hedman (University of Idaho)
Additional clues intrigue astronomers: the mu ring shows brightness variations. This phenomenon could reveal still-unexplained processes. Mark Showalter believes that only close-up images, taken by a future space mission, will allow us to understand these rings and their moons.
The exploration of Uranus has been deemed a priority in the National Academy of Sciences decadal survey. The results of the current study, published in the
Journal of Geophysical Research: Planets, pave the way for this future exploration.